The present invention relates generally to the field of optical waveguides. In particular, the present invention relates to optical waveguides that are rounded in cross-section and to methods of forming such optical waveguides. In addition, the invention relates to electronic devices that include such optical waveguides and to methods of forming such electronic devices.
Light is becoming increasingly important in the transmission of data and communications. Fiber optic cables are replacing conventional electrical cables in a number of applications. For example, optical integrated circuits (OICs) are gaining importance for high bandwidth optical interconnects on printed wiring boards.
Optical waveguides typically include a core material and a cladding layer surrounding the core material. Optical radiation propagates in the core material and is contained by the cladding layer, which has a lower index of refraction than the core material. Waveguides may be used individually or as an array supported on a substrate, and typically transmit optical radiation across a substrate surface. The waveguides often perform a passive function on the optical radiation so as to modify the output signal from the input signal in a particular way. For example, splitters divide an optical signal in one waveguide into two or more waveguides; couplers add an optical signal from two or more waveguides into a smaller number of waveguides; and wavelength division multiplexing (“WDM”) structures separate an input optical signal into spectrally discrete output waveguides, usually by employing either phase array designs or gratings. Spectral filters, polarizers, and isolators may be incorporated into the waveguide design. Waveguides may alternatively contain active functionality, wherein the input signal is altered by interaction with a second optical or electrical signal. Exemplary active functionality includes amplification and switching such as with electro-optic, thermo-optic or acousto-optic devices.
Various methods of manufacturing planar waveguides have been proposed. For example, waveguide core formation by depositing a bulk waveguide material on a substrate, followed by standard photolithography and etching processes using a photoresist on the bulk waveguide layer has been proposed. In an effort to decrease the number of processing steps and cost, the use of a photoimageable core layer in forming the waveguides has also been proposed. The core structures formed from these processes are generally square or rectangular in cross-section along the length of the waveguide. Such geometries, however, result in poor optical and insertion loss characteristics as a result of the geometrical mismatch between the waveguides and the cylindrical optical fibers to which the waveguides are typically coupled. This geometrical mismatch results in extra coupling loss at both input and output coupling ports, especially for multimode devices. Accordingly, provision of a waveguide having a core that is rounded and, more preferably, substantially circular in cross-section is desirable in order to improve light-transmission properties while reducing insertion loss due to optical fiber/waveguide coupling.
Syms et al, in Reflow and Burial of Channel Waveguides Formed in Sol-Gel Glass on Si Substrates, IEEE PHOTONICS TECH. LTTRS., Vol. 5, No. 9 (September 1993), discloses a process for fabricating waveguides by depositing a planar bilayer sol-gel glass film on a silicon substrate and etching the film to form ridge waveguides. Furnace heating is then used to melt the core and cladding glasses resulting in a smooth radius at the junction between the core and cladding. While the reflowed waveguide may provide improved loss characteristics when compared with a square or rectangular waveguide, the junction between the core and cladding is not ideal as the core is caused to spread out over the substrate.
U.S. Pat. No. 5,846,694, to Strand et al, discloses methods for manufacturing a waveguide that is nearly circular in cross-section. In the disclosed methods, a layer of an opposite material that a molten waveguide material will wet is patterned to form pedestals for a waveguide. A waveguide material is next deposited and patterned using photolithography and etching processes. Heat is applied to reflow the waveguide precursors into near-circular cross-section waveguides that sit atop the pedestals. This method has drawbacks in requiring multiple steps to define both the pedestal and the waveguide.
There is thus a need in the optoelectronics industry for improved methods of forming optical waveguides having enhanced optical and insertion loss characteristics in comparison with known, square and rectangular waveguides, as well as for waveguides formed therefrom. As well, there is a need in the art for electronic devices that include such waveguides as well as methods of forming such electronic devices.